This invention relates to improvements to digital, electrical length measuring instruments of the type comprising a bow, an anvil mounted on the bow, a measuring bolt continuously slidable with respect to the anvil in a longitudinal direction, and a transducer which comprises means for sensing the longitudinal position of the bolt with respect to the bow.
A wide range of such length measuring instruments are known to the art. For example, in German DE-AS No. 24 13 997 there is described a bow-type measuring instrument which utilizes a translatorialy operating measurement transducer which is responsive to the relative movement of two measuring surfaces and acts to measure the separation between the two measurement surfaces and to display this measurement in digital form. In the disclosed instrument, the relative movement of the two measurement surfaces is carried out by means of a spindle-nut arrangement, as is usual in the case of screw-type bow measuring instruments. The contact pressure with which the two measurement surfaces are biased together is adjustable by means of a ratchet mechanism.
A similar arrangement is described in German DE-OS No. 29 25 069; however, in this specification the position measurement is implemented with a rotationally operating measurement transducer.
Such digital, electrical, bow-type measuring instruments have a disadvantage that they are cumbersome to handle if objects with widely differing lengths within a prescribed measuring zone are to be measured. In such cases, large scale adjustments of the position of the measuring screw must be made laboriously by means of a spindle with low pitch.
Another type of digital, electrical, length measuring instrument known to the art utilizes direct translatory shifting of the measuring bolt to allow relatively rapid adjustment of the position of the measuring bolt within its measuring range. In these instruments, the measuring bolt is separated from the anvil by an operator by pressing the bolt against a restoring force provided by a spring. The spring also delivers a largely constant measuring force. In this context, three documents should be mentioned: German DE-OS No. 27 31 294, Swiss Pat. No. 527 407, and U.S. Pat. No. 4,215,480. Length measuring instruments of this second type overcome to a large extent the drawbacks of the first mentioned group with respect to rapid adjustment of the position of the measuring bolt within the measuring range. However, in every day workshop use the above described examples of measuring instruments of the second type present another serious drawback, which up to now has severely limited the practical uses of these hand measuring instruments in every day operation. In the measurement of heavy or fixed objects, for example work pieces clamped in machine tools, it is often not possible in practice to hold the measuring instrument by hand in such precise orientation with respect to the surface to be measured that an unambiguous measurement result is readable on the instrument display. Even the slightest tilting of the hand measuring instrument leads to a displacement of the measuring bolt and thereby to an altered measurement value. This alteration in the measurement value is perceived by an ever changing display. Small objects that can be clamped between the measurement surfaces of the anvil and the measuring bolt can be measured exactly only if the restoring force of the spring which biases the measuring bolt against the anvil is great enough to hold the object being measured firmly between the measuring surfaces.